This invention relates to an apparatus for feeding individual sheets over a transparent platen. The apparatus is particularly adapted for use as a document feeder for a reproducing machine.
Numerous document feeders for use with reproducing machines are known as exemplified by U.S. Pat. Nos. 3,499,710, granted Mar. 10, 1970, to Sahley; 3,556,512, granted Jan. 19, 1971, to Fackler; 3,674,363, granted July 4, 1972, to Baller, et al, and 3,790,158, granted Feb. 5, 1974, to Summers, et al. These patents are representative of the broad prior art in this area directed to document feeders for placing a document on a transparent viewing platen for viewing by the optics of a reproducing machine for making copies of the document. The first named patent shows the use of friction rolls for transporting the document over the platen, and the remaining patents show the use of belt type transport devices.
An alternative type document feeder for use with a reproducing machine is that set out in U.S. application Ser. No. 367,996, filed June 7, 1973, to Hoppner et al., now U.S. Pat. No. 3,900,258. In accordance with the subject application a scanning optical system is fixed adjacent one margin of a transparent viewing platen and a document feeder is positioned overlying the optics to advance documents past the fixed optical system at a speed synchronized to the speed of an image receiving member. This type of feeder is now employed commercially in the Xerox 3100/LDC copier.
A problem which sometimes occurs with respect to document feeders for moving original exposure is a skips and smears copy defect. For example, the document feeder of the above-noted application moves the document at a synchronized speed with respect to the photosensitive surface or other image receiving surface to provide a flowing light image of the original which is projected on the photosensitive surface. The document feeder of the above-noted application includes a sliding carriage having a first set of pinch rolls which advance the document over the viewing portion of the transparent platen and a second set of feed rolls overlying the platen which receive the lead edge of the sheet and then act upon the sheet to continue its transport after the trailing edge has left the pinch rolls. As the lead edge of the sheet intercepts the rolls overlying the platen, there can be a non-synchronous movement of the sheet which results in a skip or smear in the image on the resulting copy sheet.
The non-synchronous movement can be caused by a momentary hesitation of the sheet as it enters the pinch between the feed rolls and the platen. This hesitation can be reduced by reducing the normal force exerted by the rolls against the platen in the manner set forth in U.S. application Ser. No. 464,725, filed Apr. 26, 1974 to Van Buskirk now U.S. Pat. No. 3,909,129.
It has now been found that for a sliding document feeder carriage having feed rolls formed of a conventional material such as polyurethane the forces exerted on the carriage by the frictional engagement between the feed rolls and the platen can be greater than the latching force designed to hold the carriage in its operating position. This can result in a displacement of the carriage in the direction opposite to the direction of document travel. The carriage maintains this position until a document enters the nip between the feed roll and the platen. The coefficient of friction between the glass platen and the document is lower than the roll to platen friction and, therefore, the force driving the carriage out of position is substantially reduced. As a result the carriage returns to its operating position at a velocity higher than the acceptable document velocity thus creating an unacceptable skip in the resulting image on the copy sheet. This problem can be further aggravated due to the humidity sensitivity of the feed roll material. It has been found that in relatively high humidity, polyurethane rolls absorb moisture which further increases its coefficient of friction relative to the platen thereby making the problem worse.
A partial solution to this problem is to more securely latch the carriage in its operative position as by the use of a mechanism as described in U.S. patent application Ser. No. 482,739, filed June 24, 1974, to VanBuskirk now U.S. Pat. No. 3,944,365. However, even this approach is not a full answer to this problem due to the mismatch between the roll to platen and document to platen coefficients of friction.
In accordance with this invention this mismatch in coefficient of friction is substantially reduced through the use of a multi-component feed roll material most preferably comprising polyurethane impregnated with silicone oil. This material also exhibits a greater degree of moisture insensitivity as compared to polyurethane alone and other similar materials.
Silicone oil impregnated silicone rubber has been employed in fuser rolls as in U.S. Pat. No. 3,731,353, to Artl. The use of porous rolls impregnated with a nonvolatile liquid to prevent off-setting when used as a transport roll for forwarding sheets having on a surface a non-dry tacky material is described in United Kingdom Pat. No. 1,257,498, published Dec. 22, 1971.